In mining, blasting systems are used for sequentially triggering explosive charges placed in bore holes at a working face. Each explosive charge is ignited by an associated trigger stage, and all trigger stages are connected in cascade with a pulse source via a control line. The control line often takes the form of a flat cable usually including four wires; some circuits require only three or even only two wires. Examples for such sequential blasting systems are described in U.S. Pat. No. 5,571,985 and German Patent No. 4,433,880.
The circuit components of each trigger stage are usually assembled on a printed circuit board enclosed in a protective casing. The circuit board is provided with contact points for connecting, by soldering, the incoming control line, the outgoing control line, and branch lines which lead to one or more explosive charges. This results in two soldering points for each individual wire of the control line within the area of each trigger stage, which not only necessitates corresponding expenditure in manufacture but also involves a risk of faulty contacts.
In sequential blasting systems, a single faulty contact may result in only part of the explosive charges being triggered or in explosions being simultaneously initiated at a plurality of locations, depending or the type of circuitry employed and on the location of the fault. In any case, the propagation of the explosion pressure wave, which has been designed by suitably placing of the blasting system, is disturbed and the efficiency of the sequential blasting system is at least substantially reduced. Moreover, an interruption of the control line within the region of its connection to a trigger stage may be caused by tensile stress exerted on the control line, which frequently occurs due to cough handling, particularly in underground mining.